Substrate Specificity of Human Cytochrome P450 (CYP) 2C Subfamily and Effect of Azole Antifungal Agents on CYP2C8

PURPOSE: The metabolic activities of aminopyrine N-demethylation and tolbutamide methylhydroxylation by the human hepatic cytochrome P450 (P450 or CYP) 2C subfamily were compared and the effects of azole antifungal agent on the drug-metabolizing activity of CYP2C8 were investigated. METHODS: Aminopyrine N-demethylation and tolbutamide methylhydroxylation by CYP2C8, CYP2C9, and CYP2C19 were determined by the previous reported methods. The effects of five azole antifungal agents, fluconazole, itraconazole, ketoconazole, miconazole, and voriconazole, on the aminopyrine N-demethylation activity by CYP2C8 were investigated. RESULTS: With regard to aminopyrine N-demethylation, CYP2C19 had the lowest Michaelis constant (K-m) and CYP2C8 had the highest maximal velocity (V-max) among the CYP2C subfamily members. The V-max/K-m values for CYP2C8 were the highest, followed by CYP2C19. For tolbutamide methylhydroxylation, the K-m and Vmax for CYP2C19 were three and six times higher than the corresponding values for CYP2C9, and the Vmax/Km value for CYP2C19 was twice that for CYP2C9, whereas hydroxylated tolbutamide formed by CYP2C8 was not detected. Fluconazole, itraconazole, and voriconazole at a concentration of 2 or 10 mu M neither inhibited nor stimulated CYP2C8-mediated aminopyrine N-demethylation activity at substrate concentrations around the K-m (5 mM). However, ketoconazole and miconazole noncompetitively inhibited CYP2C8-mediated aminopyrine N-demethylation with the inhibitory constant values of 1.98 and 0.86 mu M, respectively. CONCLUSION: These results suggest that ketoconazole and miconazole might inhibit CYP2C8 clinically.